First Primates

Posted 07.09.08

NOVA scienceNOW

As inhabitants of Earth, we humans are relative newbies. In fact, our branch of the evolutionary tree may have split from that of the apes only about six million years ago. But what if we look further back in our primate family tree? What would we find? As correspondent Peter Standring reports, the latest research is revealing that our origins may have been quite a bit humbler than we may have thought.

Transcript

First Primates

NEIL
DeGRASSE TYSON: As inhabitants of
Earth, we humans are relative newbies. In fact, our branch of the evolutionary
tree may have split with these apes only about 6,000,000 years ago.

But
what if we look further back in our primate family tree? There must have been
some great and wise ancestor who founded this wonderful line of creatures,
right? Well, as correspondent Peter Standring reports, the latest research is
revealing that our origins may have been quite a bit humbler than we thought.

PETER
STANDRING: The Badlands of Wyoming:
some of the largest dinosaur bones, ever, were found right here. But University
of Florida paleontologist Jonathan Bloch is hunting for a set of bones that are
nothing like the giant bones of T-Rex.

PETER
STANDRING: Tiny mouse-sized bones,
buried in limestone, that just might be the fossil remains of our earliest
primate ancestors.

An
age-old mystery surrounds the origin of primates. No one knows exactly where we
come from or how we got our evolutionary start.

Here's
what we do know: giant dinosaurs once ruled this basin, where they dined freely
in a lush forest. But then, around 65 million years ago, the dinosaurs die off
when a massive comet slams into the planet. Ten million years later, something
extraordinary happens. The fossil record suddenly shows a new kind of mammal,
with unique characteristics: the primate, our ancient ancestors.

So
what is a primate? What is it that separates us from the rest of the
evolutionary pack? Well, maybe it's our good looks or our superior
intelligence.

The
truth is brain size does come into play. We primates, even Noah here, have
larger brains than our mammal relatives. It's a feature that evolved to help us
learn complex social behavior and how to do things like make tools or even
outwit our prey.

We
also developed forward-facing eyes with stereo vision. It's a feature that
allows us to judge the world around us in 3D. Over time, we also developed the
ability to leap, basically to jump from branch to branch, where grasping hands,
or in Noah's case, grasping feet, equipped with nails instead of claws, enable
us to reach that tasty piece of fruit.

Our
earliest ancestors developed these unique characteristics, some time after the
extinction of dinosaurs. The question is, "When and why?"

So
let me get it straight. If the dinosaurs became extinct 65 million years ago,
and then primates suddenly appeared around 56 million years ago, what happened
in between? I mean that's almost 10 million years that's unaccounted for.

JONATHAN
BLOCH: Right. That's the $6,000,000
question. And I don't think they just appeared on the face of the planet, they
evolved.

PETER
STANDRING: But from what? I mean,
something the size of a mouse?

JONATHAN
BLOCH: Exactly.

PETER
STANDRING: Jonathan believes the
evidence to support his theory and help solve this ancient primate mystery can
be found here, hidden inside the limestone of the Bighorn Basin.

JONATHAN
BLOCH: A tiny little piece of broken bone
can connect up with an entire skeleton of a mammal.

This
looks like a pretty good limestone. It should be...should be full of fossils,
but we really won't know until we get it back to the lab.

You
see a tiny little piece of bone, and you hope that there's more inside, you
have no guarantees, so it's a little bit of a gamble.

JONATHAN
BLOCH: These limestones allow us a window
into that world that we've never had before.

PETER
STANDRING: The world of the
earliest primate. It will take a 2,000-mile drive back to his lab in
Gainesville, Florida, and a year of painstaking work, to find out if Jonathan's
gamble will pay off.

Back
in his lab, Jonathan, along with graduate student Doug Boyer, gets to work.
Their goal? To free the delicate bones from the rock-hard stone. They begin by
placing the limestone under a microscope.

JONATHAN
BLOCH: That immediately starts to open up
the world of the block. We identify all of the bone that's outcropping on the
surface.

PETER
STANDRING: Doug carefully coats the
tiny bones with plastic to protect them from the powerful acid bath they're
about to take.

DOUG
BOYER (Graduate Student, Stony Brook University): We leave the block in acid for, at the most, two to
two and a half hours, and that'll remove about a millimeter-thick rind of
limestone.

JONATHAN
BLOCH: We repeat the process, again and
again and again and again, until all of the bone is exposed.

PETER
STANDRING: Much to their surprise
they find hundreds of tiny bones. But success poses a new problem.

JONATHAN
BLOCH: It's not always obvious which bones
go to what animal, and so the only way to document that is by creating a little
archeology site, a map of all the bones.

PETER
STANDRING: Doug devises a method to
meticulously document the relationship between each and every bone. The process
will take months, but when complete, it will reveal far more than they ever
anticipated: dozens of tiny mammals never before seen, including these three
extraordinary skeletons.

And
what are these?

JONATHAN
BLOCH: These are plesiadapiforms,

PETER
STANDRING: Plesiadapiforms are tiny
mouse-like creatures that lived during the mysterious 10-million-year period
between the extinction of dinosaurs and the appearance of primates. It's a very
diverse group, with more than 120 species, including these three.

JONATHAN
BLOCH: They represent the most complete
skeletons of plesiadapiforms known in the world.

PETER
STANDRING: An extraordinary find,
for sure, but will they help Jonathan solve this primate mystery? Are
plesiadapiforms our earliest ancestors?

JONATHAN
BLOCH: If we look here, this nail-like
structure makes you think, because the presence of a nail is a hallmark
characteristic of living primates.

PETER
STANDRING: This is an enlarged
image of the extraordinary nail Jonathan found. Next to it, the claw he
expected–a startling difference.

JONATHAN
BLOCH: This nail might actually be the
first nail in the history of primate evolution.

PETER
STANDRING: Concrete evidence to
support his theory of primate evolution. Could there be more hidden within
these tiny bones?

To
find out, Jonathan enlists the help of Mary Silcox, evolutionary anthropologist
at the University of Winnipeg. She's been busy zapping primitive skulls with an
industrial-strength CAT scanner, large enough to fill an entire room. Mary
takes the skull of one of the limestone skeletons and prepares it for scanning.

MARY
T. SILCOX (University of Winnipeg):
The x-ray goes through the specimen, and
we collect 2,400 separate views, which produce a cross-sectional image.

A
structure that had been identified as just a little piece of bone in the middle
ear actually had the form of a tube. And the reason that was exciting was
because there's a structure running through the ear of particularly primitive
primates–things like lemurs–which is a tube for a large vessel that
goes to the brain.

PETER
STANDRING: A tiny tube, a tiny
nail, the evidence is mounting. But to prove his theory of primate evolution,
Jonathan still needs more. He adds another member to the team. Eric Sargis,
professor of anthropology at Yale University, and the world's leading expert on
tree shrews. Why a tree shrew expert? Scientists believe that tree
shrews–a primitive species of tiny tree-living mammals–are actually
related to early primates.

ERIC
SARGIS: Tree shrews are not primates, but they're close relatives.
They share a number of characteristics that separates them from other groups of
mammals.

PETER
STANDRING: Would plesiadapiforms
pass the ultimate primate test? Are they the first step on the primate family
tree or just another relative on the tree shrew family tree?

MARY
SILCOX: What we were interested in was to test whether or
not plesiadapiforms were the earliest primates.

PETER
STANDRING: The team goes to work
bringing together all the information they had collected independently into a
single comprehensive study: Jonathan and Doug's plesiadapiform skeletons;
Mary's scans of dozens of ancient skulls; and Eric's anatomical data on a close
living relative, the tree shrew.

ERIC
SARGIS: The way we start is by comparing all these
specimens.

PETER
STANDRING: Detail by detail,
feature by feature they combed through all the data using a numerical system to
compare and contrast.

JONATHAN
BLOCH: After we studied the different
characteristics of these animals, and reduced them down to numbers–you know,
absence of a nail is a 0, presence of a nail is a 1–we then ran this
through a computer algorithm.

PETER
STANDRING: The algorithm sifted
through the complex data in search of simple relationships: which fossils have
the same characteristics, the same numbers. Using this information, the
computer was programmed to create family trees illustrating the potential
relationships each mammal has to the next. The team expected the computer to
come up with several possible scenarios in the form of several possible family
trees. Instead, the program came up with only one.

JONATHAN
BLOCH: I was a little surprised to see it
so unambiguous.

PETER
STANDRING: This single family tree
could lead to only one conclusion.

JONATHAN
BLOCH: I think the evidence, as it stands
today, is pretty compelling that yes, in fact, these are primates.

MARY
SILCOX: Every new piece of data that we had coming out of
our study of this material seemed to be consistent with that idea.

PETER
STANDRING: Not only that. One of
the plesiadapiform skeletons Jonathan and Doug painstakingly etched out of
limestone, a species by the name of Dryomomys, turns out to be far more
primitive than the other two, possessing only one primate characteristic, the
shape of its teeth.

ERIC
SARGIS: It's sort of a transitional specimen between more
primitive things, like tree shrews, and later primates.

PETER
STANDRING: One part primate, other
parts not.

ERIC
SARGIS: I mean, it really starts to tell us something about
the base of the primate tree, what the earliest primates look like. So, if we're
one leaf on the branch, so are chimpanzees, gorillas, orangutans, among apes;
all the different monkeys in the old world and the new world; lemurs from
Madagascar; lorises and galagoes; all those animals are living today, but you
can trace it all back to a single common ancestor. And as you get closer and
closer to that common ancestor, dryomomys is one of the animals that's closest
to the base there. It's the most primitive primate skeleton ever found, to
date.

PETER
STANDRING: Jonathan had evidence to
support his theory. Primates didn't just appear on the planet, they evolved
over a 10-million-year period. And just as he thought, the earliest primates
were the size of a mouse. Still one question remains. What sparked this amazing
transformation? The team believes our ancient ancestors evolve on the heels of
a mass extinction. Without the mighty T-Rex around, the tiniest of mammals are
free to forage and explore, and they discover a world filled with flowering plants
and succulent fruit.

MARY
SILCOX: We have this sort of co-evolutionary relationship,
where fruits were evolving to get tastier for primates to eat; the primates
were then eating them and helping the plants actually spread their seeds
further.

PETER
STANDRING: With tempting fruit
growing at the end of tiny branches, our ancestors have plenty of motivation to
change. So they begin to evolve, developing long fingers for climbing trees,
specialized teeth, hands and feet, uniquely designed for grasping and eating
the tiniest, tasty berry. Over 10 million years, they slowly develop unique
characteristics that we recognize in our primate relatives and ourselves.

ERIC
SARGIS: So that if plesiadapiforms don't evolve, we're
probably not standing here talking about this right now.

Neil deGrasse Tyson is director of the Hayden Planetarium in the
Rose Center for Earth and Space at the American Museum of Natural History.

This material is based upon work
supported by the National Science Foundation under Grant No. 0638931. Any
opinions, findings, and conclusions or recommendations expressed in this
material are those of the author(s) and do not necessarily reflect the views of
the National Science Foundation.

Funding for NOVA scienceNOW is provided by the National Science Foundation, the Alfred P. Sloan Foundation, and PBS viewers.

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